1. Field of the Invention
The present invention relates to a lead-free bonding material having high heat resistance and a bonding method used in the semiconductor field, and relates, for instance, to a sintering silver paste material for obtaining a joint part with high heat resistance and excellent in thermal and electric conductivity, by strongly bonding to a member of a nonprecious metal such as Cu and Ni in a bonding process at a low temperature of 300° C. or lower and without applying pressurization, and a method for bonding a semiconductor chip by the material.
2. Background Art
The use of a solder containing lead in the electrical and electronics fields has been restricted by the environmental regulations advocated in Europe, and all solders for substrate packaging have been converted to a lead-free solder. However, a high melting-point solder (melting point >260° C.) which is used for bonding a chip of a power system device and has a high lead content is still used because no alternative material has been developed yet. For this reason, it becomes urgently necessary to develop a lead-free connecting material which is resistant to high temperatures.
In the present state, the most dominant candidate of a lead-free connecting material for the alternative high-lead solder includes an electroconductive paste material of a metal bonding type. As for this typical electroconductive paste material, there are known an Ag nano-particle paste in which Ag particles with a nanometric size are dispersed in a solvent (JP Patent Publication (Kokai) No. 2007-095510 A), and an Ag micro-particle paste in which silver particles with a micrometric size and an organic solvent are kneaded into a slurry state (International Publication No. WO 2006/126614).
The Ag nano-particle paste has an organic protective film formed on the surface so as to prevent the cohesion among Ag particles and to secure dispersibility, and is sintered at a low temperature by decomposing and removing the protective film by heating. Because the Ag nano-particle is very active, the Ag nano-particle can be bonded to Cu and Ni with a certain degree of strength if a load is applied to the Ag nano-particle and adhesion is secured, but on the condition in which the load is not applied, the bonding strength becomes very weak. In addition, because large volumetric shrinkage occurs and large voids are locally formed when the paste is sintered, it is necessary to apply a load to the paste in bonding so as to reduce bonding defects.
On the other hand, the Ag micro-particle paste easily progresses its sintering process when heated in the atmosphere (200 to 300° C.) and a bonding strength of 10 MPa or more is easily obtained even without applying the load to the paste. However, a bondable object material is limited to precious metal materials such as Au and Ag, which are not oxidized by heating in the atmosphere of 200 to 300° C., and with respect to Cu and Ni, the strength of merely several MPa or less is obtained by the bond.
Most of metallic members composing a semiconductor are made from Cu which adequately conducts electricity and heat, and the material of a face to be bonded is usually Cu. In addition, the Cu surface of the member at which oxidative degradation by heating becomes a problem is plated with Ni, and the material of the face to be bonded in thus Ni.
The high-lead solder adequately wets Cu and Ni (adequately receptive), and can bond a semiconductor chip to these members without any problem. When the member is bonded by using a highly electroconductive Ag paste of the alternative high-lead solder on the condition of applying no pressure, the metallic member is necessary to be plated with a precious metal, which causes a problem of increasing a member cost.
When a Cu or Ni member is bonded by using the Ag micro-particle paste on the condition of applying no pressure, the member is not bonded to sintered Ag because the surface of the member is oxidized in a heating process, which causes a problem that a necessary strength cannot be obtained.
When the Ag micro-particle paste is heated and baked in a non-oxidizing atmosphere in order to prevent the oxidization of the member, an organic component which deposits on the surface of the Ag micro-particle cannot be decomposed and removed at a temperature of 500° C. or lower, which accordingly causes a problem that the sintering between Ag particles does not progress and the members cannot be bonded.
A first object of the present invention is to provide a sintering type Ag micro-particle paste material which can metallically bond a semiconductor chip to even a member of a nonprecious metal such as Cu and Ni with a strength of 10 MPa or more, under the bonding condition of applying no pressure. A second object of the present invention is to provide a bonding method for bonding a semiconductor chip to Cu or Ni with a strength of 10 MPa or more by using the Ag particle paste under the condition of applying no pressure.